Flashphoner has built-in load statistics interface.
You can get current calls and connects count over HTTP.
Using it, you can implement custom load balancing scheme between Flashphoner server nodes.
Flashphoner signalling has been implemented over SIP.
And Flash-to-Flash calls uses SIP signalling also.
So, you need just add Flashphoner server node into existing load balancing scheme for horizontal scalability your system.
